check for ip in reverse proxy header

cmd/whoami/main.go

@@ -20,8 +20,14 @@ func domainFromIP(target net.IP) string {
 
 // handle HTTP requests
 func handler(w http.ResponseWriter, r *http.Request) {
+	// get client's ip address
 	ip, port, err := net.SplitHostPort(r.RemoteAddr)
 	_, _ = port, err	
+	// check if we're behind a reverse proxy
+	xForwardedFor := r.Header.Get("X-Forwarded-For")
+	if xForwardedFor != "" {
+		ip = xForwardedFor
+	}
 	parsedIP := net.ParseIP(ip)
 	// return domain for IPv6 only
 	if parsedIP.To4() == nil {